Solar Activity

The intense magnetic fields produced in the solar interior influence the physical structure of the photosphere, chromosphere, and corona in a complex and time-varying way described collectively as solar activity.

Sunspots, Faculae, and Flares. The magnetic fields emerge at the visible layers as toroidal loops of magnetic flux up to 100,000 km (62,000 mi) in diameter. Their most obvious effect at the photosphere is to produce the dark sunspots and the bright areas called faculae that constitute active magnetic regions. If, as believed, the intense radially directed fields inhibit convection and reduce the efficiency of the dominant heat-transport process to the photosphere, the low temperature and relative darkness of sunspots is explained. How intense fields can also produce a net facular brightening under similar circumstances is unclear.

An active region grows in horizontal extent as the loop emerges from less than 5,000 km (3,100 mi) across to more than 100,000 km within 10 days. During this period of rapid growth the probability of a spectacular eruption, called a solar flare, is highest. A large flare is marked by a rapid brightening within a few minutes of a considerable area of the active region by a factor of 5 to 10, as seen in chromospheric radiations such as the H alpha line of hydrogen. Only the very largest flares can be detected in integrated white light against the bright photosphere. The most violent and spectacular effects of the eruption, however, take place in the corona above. There, a set of the magnetic loops above the spots and faculae may increase their brightness in X rays and ultraviolet light by a factor of 100 or more. Charged particles are accelerated to relativistic energies, and strong centimeter-wave emission is generally detected.

Some flares also produce powerful meter-wave radio bursts, and large volumes of hot plasma, called sprays, are often ejected into space at speeds exceeding the escape velocity of 617 km/sec (380 mi/sec) from the solar gravitational field. The cataclysmic event decays more slowly, over a few hours, after liberating up to 1032 ergs of energy. The source of this power is most likely the rapid dissipation of electric currents flowing in the corona. Theoretical models suggest that this energy could be released fast enough to explain the rapid brightening in X rays and ultraviolet light, the acceleration of charged particles, and the ejections of plasma that are observed during flares.

Sunspots generally last a few weeks, with the most persistent large spots surviving for two to three months. The faculae continue to mark an active region for somewhat longer. Eventually, it appears that the random motions of convection near the photosphere disassemble the magnetic-flux loop and disperse it into smaller elements distributed over the surface.

Away from the active regions less extended fields of comparable intensity (1,000 to 2,000 gauss) are measured, but they are confined to a polygonal network that coincides with the edges of the supergranular convective cells mentioned above.

Loops, Prominences, and Coronal Holes. Above the photosphere the magnetic fields over an active region can be seen by their effect on the distribution of temperature and density in the chromosphere and corona. There, prominent loop-shaped structures seen in X rays and ultraviolet light show how the field lines extend to 100,000 km (62,000 mi) or more above a spot and then connect back to the photosphere, generally within the same extended patch of activity. In other regions of the corona immense sheets of relatively cool (10,000 K as opposed to the 1 to 3 million K of the corona) condensed plasma, called prominences, are supported at heights up to 200,000 km (124,000 mi).

In certain large areas, called coronal holes, the coronal emission is significantly depressed, indicating a low density of the million-degree plasma. Studies indicate that in these regions the field lines continue radially outward and do not form closed structures, as in loops or prominences. Models show that the hot corona can then flow out into interplanetary space more easily, leaving a deficit of coronal material. Such holes are particularly common at the solar poles, where no active regions with closed fields are observed.

Solar-Activity Cycle. Solar activity exhibits a cycle over a period of about 22 years, the most easily observed feature of which is the approximately 11-year variation in the number of sunspots. At the beginning of a new cycle the first groups emerge at latitudes between 35 and 40 degrees; their magnetic polarity is opposite to that of the last groups of the preceding cycle in the hemisphere concerned. Thus two consecutive 11-year cycles are required to return to a given level of spot number and a given polarity.

The 11-year cycle seems to have been fairly regular over the past century and longer, but historical evidence indicates that between about 1640 and 1710 - the so-called Maunder Minimum - hardly any spots were visible at all. Long-term irregularities in solar activity are of practical interest because solar fluxes of charged particles and ultraviolet radiation are directly controlled by the level of activity through active regions, flares, and coronal holes. Variations in these emissions are known to affect the upper atmosphere and also the concentration of stratospheric ozone.

The Sun's total input of heat and light at the top of the Earth's atmosphere, known as the solar constant, has been measured and shown to vary over the solar cycle. Observations of satellites since 1978 have demonstrated that the Sun is about 0.05% to 0.1% brighter at the peak of sunspot activity than during a sunspot minimum. The reason seems to be that the area of the solar photosphere covered by the bright magnetic faculae increases, and their great light output more than compensates for the dimming of the Sun caused by the dark sunspots. The possible effect of variations in the solar constant is of great interest in interpreting climate change on Earth.

The mechanism underlying the solar-activity cycle is also the subject of much research. It appears most likely that the intense magnetic fields of spots and faculae originate in the intensification of a weaker general field by large-scale plasma motions driven by convection in the Sun's interior. In such MHD dynamo models the intense fields appear then to be dissipated by the smaller-scale turbulence seen at the solar photosphere, although the dynamics of the solar cycle still remain to be worked out in detail.